In order to attain an acceptable level of safety in the operation of a nuclear power plant or similar installation incorporating a nuclear reactor, it is necessary to provide an emergency core cooling system (ECCS). The principle function of the ECCS is to dissipate the decay heat in the core after the reactor is shut down if there is a failure in the main coolant system. Such action is necessary because the core of the reactor continues to produce heat, known as decay heat, even after the removal of the moderator makes the core subcritical. The decay heat is sufficient to produce temperatures that ultimately would melt the cladding and the fuel and destroy the integrity of the pressure vessel. High temperatures also create substantial quantities of free hydrogen with the resultant possibility of a catastrophic explosion.
A conventional safety system frequently utilized in water-cooled reactors incorporates sufficient pumping capacity to pump larger quantities of cooling water into the reactor vessel with sufficient excess capacity to compensate for leakage through a break in the main coolant system.
The use of cooling water has numerous disadvantages for which there has been no effective solution. A back-up system for shutting down the reactor must be provided because the water acts as a moderator making the core critical. In some cases, a borated solution is used for this purpose. The system must move large quantities of water very quickly after it is activated and therefore numerous relatively vulnerable components must function properly and in sequence for the system to be effective. For example, the power required to drive the pumps must be provided by an auxiliary generator. Normally, Diesel generators are utilized for this purpose. Therefore, the Diesel engines must first be started before pumping can commence. The starting of Diesel engines requires significant time and is not fool-proof. After engine starting, the pumps for the cooling water must be brought up to speed. Since there are relatively large rotating masses involved, this requires significant time. During the time that the machinery is being put on line, the core is increasing in temperature. The temperatures may be so great that fuel rod swelling and other deformation of the core may have taken place which reduces the penetration of the water and thereby reduces its capacity to provide sufficient cooling. In addition, the high temperatures flash much of the initial water into steam creating a back pressure that prevents further penetration of the water through the core. Since the increase in temperature in the core reduces the ability of the water to cool, an unstable situation is created which is undesirable in a safety system. Further, the quenching action of the water on embrittled fuel rods at elevated temperature may cause ultimate structural failure of those critical elements. Finally, an elaborate control system must be provided to maintain the operational stability of the system during the emergency cooling action.
Therefore, it is desirable to have an emergency core cooling system with enhanced reliability.